1. Field of the Invention
The present invention relates to a novel recording medium and a spectral reflectance changeable alloy (i.e. an alloy capable of changing its spectral reflectance). More particularly, the invention is concerned with an alloy which can be used as medium for recording or displaying information, as well as materials of sensors, through a change in the spectral reflectance as a result of a change in the crystal structure which takes place in response to light or heat energy applied to the alloy.
2. Description of the Prior Art
In recent years, there is an increasing demand for higher density and digitization of information recording, which in turn gives a rise to the demand for development of various information recording/reproducing technics. In particular, optical disc, which is capable of recording, erasing and reproducing information in cooperation with a laser beam, exhibits a higher density of recording than conventionally used magnetic discs, as described in "Industrial Rare Metal No. 80, 1983" (optical disc and material). Thus, the optical disc will become one of the most effective and practical information recording/reproducing system in the coming future.
The reproducing device making use of laser beam has been put into practical use under the name of "Compact Disc") (abbreviation "CD").
On the other hand, the presently available systems which enable recording of information can be sorted into two types: an unerasable type and a rewritable type. The unerasable type permits writing only once and does not allow erasure, while the rewritable type permits repetitional writing and erasure. The recording method of the unerasable type is to form fine convexities and concavities in the medium by breaking or shaping the medium by means of a laser beam, while the reading of the recorded information is conducted by making use of a change in the light reflectance due to interference of the laser beam caused by the minute concavities and convexities. For instance, it is well known to form convexities and concavities by melting or sublimation of a recording medium made of, for example, Te or its alloy, by means of a laser beam. This type of recording medium, however, involves problems such as toxicity.
On the other hand, photoelectromagnetic materials are major material as the rewritable type medium. The recording with this material is conducted by inverting local magnetic anisotropy in the medium at a temperature around Curie point or compensation point by the energy of a light, while the reproduction is conducted by a rotation amount of plane of polarization caused by the magnetic Faraday effect or magnetic kerr effect brought about by the polarized incident light. This recording/reproducing method is considered as being the most promising method using the medium of rewritable type and an intense study is being made with a prospect that this method will put into practical use within several years. Unfortunately, however, no material has been found which would provide a sufficiently large rotation amount of the plane of polarization. At the present stage, it is still impossible to obtain sufficiently high level of output such as S/N and C/N, despite various efforts such as lamination of the medium material.
Another known system of rewritable type makes use of a change in the reflectance caused by a reversible phase change between amorphous state and crystalline state of the recording medium material. An example of such a recording medium material is obtained by adding trace amounts of Ge and Sn to TeO.sub.x, as disclosed in "National Technical Report Vol. 29, No. 5 (1983)". This system, however, suffers from a problem in that the instability of phase at normal temperature due to a low crystallization temperature of the amorphous phase effects on the reliability of a disc.
Meanwhile, Japanese Patent Application Laid-Open Publication No. 140845/82 discloses an alloy which exhibits a change in color. This alloy has a composition consisting essentially of 12 to 15 wt % of Al, 1 to 5 wt % of Ni and the balance Cu, and changes its color reversibly from RED to gold and vice versa across the martensite transformation temperature. The martensite transformation is a phenomenon which inevitably takes place when the temperature of the material has come down. That is to say, the color which is obtained when the material temperature is above the martensite transformation temperature cannot be maintained when the material temperature has come down below the martensite transformation temperature. To the contrary, the color obtained when the material temperature is below the martensite transformation temperature is changed into another color when the material temperature is raised above the martensite transformation temperature. Therefore, two different colors occuring above and below the martensite transformation temperature cannot be simultaneously obtained at an equal temperature of the material. This means that this principle making use of change in the color cannot be applied as a recording material.